Hydrodynamic noise from an air-water interface is often encountered in marine engineering structures. It involves a complex mechanism, various forms of sound sources and many factors affecting the propagation. It interacts with the free surface and is affected by cavitation and water-air-bubble mixed flow, which has a negative impact on the concealment of the ship, giving this line of inquiry strong research significance. This paper introduces a noise calculation method and its applications in terms of free surface noise, cavitation noise and water-air-bubble mixed flow noise respectively, and describes their computational methods and key issues. The solutions currently in use are also introduced. Finally, future research directions are proposed.
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In order to explore the influence of the free surface and free end on the flow field around typical bluff bodies, the flow field around a finite-length cylinder with a free surface is studied.
Based on the delay detached-eddy simulation (DDES) approach and piecewise linear interface calculation (PLIC) method, our in-house solver naoe-FOAM-SJTU is adopted to carry out numerical simulations.
The results show that the existence of the free surface and free end increases the lift and drag of local positions, and delays the occurrence of flow separation on the cylinder's surface. Compared with the deep draft region, the recovery of streamwise velocity near the free surface is delayed, and the transverse velocity tends to move outward. The deformation of the free surface generates many small vortices, and the twisted vortex at the free end restrains the development of the Kármán vortex street to a certain extent.
This study shows that the current numerical methods can accurately capture this complex flow field. At the same time, the existence of the free surface and free end significantly changes the distribution of the flow field in the draft direction.
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